Gas ignition apparatus

ABSTRACT

A spark ignition system for a heater-type ignitor is disclosed. Current through the ignitor is transformed into a voltage which is utilized to operate a timing circuit which, in turn, causes operation of a valve permitting the flow of fuel to a burner. Operation of the valve can occur only after the expiration of a predetermined period of time and the presence of a predetermined minimum voltage to the ignitor thus ensuring that the ignitor has reached the ignition temperature of the fuel before the valve is permitted to open.

TECHNICAL FIELD

The present invention relates to apparatus for igniting gas flowing froma burner and more particularly to apparatus which ensures that aheater-type ignition device has reached ignition temperature before gasis allowed to flow from the burner.

BACKGROUND ART

In many gas burner applications it is desirable or necessary to ensurethat the ignition device is fully operable before gas is allowed to flowwithin the system. In essence, the gas ignition device is "proven" priorto gas being allowed to flow through the system. Various approaches havebeen taken in order to "prove" the ignition device prior to gas flow.For example, one approach requires a visual recognition or detection ofthe spark from a sparking ignition device prior to allowing gas flow. Ithas been found that the detection of such a spark is very difficult inthe presence of ambient light, and the detection means must therefore beshielded from external light. Another approach is based on the acousticrecognition, rather than the visual recognition, of the spark. Hereagain, it has been found that it is very difficult to shield againstexternal noise and the detection source must be capable of detecting theparticular sound of the spark. Still another approach is based onproving the existence of energy pulses in the spark generating circuit.This approach has inherent problems since it is possible to have suchpulses without an actual spark. Still another approach is based uponmeasuring the electrical resistance of a heater-type ignition device andcomparing same to a reference resistance. In this case, the gas valve isnot allowed to open until the resistance of the ignition deviceapproximates that of the reference resistance. It has been found thatthis reference comparing technique requires complex circuitry which issubject to failure and has inherent problems caused by aging of thereference resistance and/or other circuit components. Thus, each of theprior art approaches of ensuring that the ignition device is fullyoperable before gas is allowed to flow in the system has some inherentproblems.

Because of the foregoing limitations and problems associated with theprior art approaches of ensuring that ignition device is fully operablebefore the gas valve is allowed to open, it has become desirable todevelop a simple, fail safe ignition system which prevents gas flow tothe burner until the heater-type ignition device has reached ignitiontemperature.

SUMMARY OF THE INVENTION

The present invention solves the problems associated with the prior artdevices and other problems by placing the heater-type ignitor in serieswith the primary coil of a transformer. The combination of the primarycoil of the transformer and the heater is provided with electrical powerfrom an AC power source which will usually be 117 volts, but is notlimited to a 117 volt AC source. The secondary coil of the transformeris connected to a half-wave rectifier and a filter capacitor whichproduces a DC voltage level having moderate ripple. This DC voltagelevel is then clamped by a zener diode producing a relatively constantDC output voltage which is applied to a gas valve starting circuitcausing negligible loading of the DC voltage. The gas valve startingcircuit opens the gas valve only after it has received power for apredetermined period of time. After the expiration of the predeterminedperiod of time, the starting circuit operates a relay which, in turn,operates the gas valve permitting gas to flow within the system. Therelay actuating circuit is adjusted so that no gas can flow unless theline voltage at that time, i.e., end of the heating cycle, is at least acertain reference level, e.g., 95 volts for a 117 volt AC power source.In this manner, voltage must be applied to the heater for thepredetermined period of time and the voltage must be sufficient beforethe gas valve is allowed to operate, thus ensuring that the heater hasreached ignition temperature before actuation of the gas valve. Inanother embodiment of the system, the actuating circuit for the relaywhich transmits power to the heater is also adjusted so that it is notactuated unless the line voltage is at least a certain reference level,e.g., 95 volts for a 117 volt AC power source, thus ensuring sufficientvoltage to the heater at the start of the heating cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of the circuit utilized by the presentinvention.

FIG. 2 is a drawing of the wave shape produced by the half-waverectifier connected to the transformer secondary coil of the presentinvention.

FIG. 3 is a schematic drawing of the circuit utilized by the presentinvention in conjunction with a start circuit, a flame rectificationcircuit and a gas solenoid valve.

FIG. 4 is a schematic drawing of the circuit used to actuate theheater-type ignition device incorporating an adjustment to prevent powerfrom being supplied to the heater unless the power source voltageexceeds a reference level.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings where the illustrations are for thepurpose of describing the preferred embodiment of the present inventionand are not intended to limit the invention hereto, FIG. 1 is aschematic diagram of a power circuit 10 for an ignitor. As such, thecircuit 10 includes a voltage to current transformer, shown generally bythe numeral 12, having its primary coil 14 connected in series with aheater 16, such as a silicon carbide ignitor. The combination of theprimary coil 14 of the transformer 12 and the heater 16 is providedpower by a 117 volt AC source connected across the input terminals 18and 20. The secondary coil 22 of the transformer 12 is connected to adiode 24 and a capacitor 26. A zener diode 28 is connected in parallelacross the capacitor 26, and the output of the circuit 10 is takenacross the terminals of the zener diode 28. The output of this circuit10 is connected to a start circuit of a complete ignition system whichmight take one of many different forms, depending upon the desiredapplication. The start circuit must, however, draw low current, i.e.,less than a few milliamperes.

The primary coil 14 of the transformer 12 consists of a few turns ofwire having a cross-section sufficient to carry about 5 amperes maximumcurrent. For example, in one embodiment the total resistance of theprimary coil 14 is approximately 0.03 ohms and the inductive reactanceis on the order of 0.1 ohms at 60 Hz resulting in a voltage drop ofapproximately 0.5 volts across this coil 14. The secondary coil 22 ofthe transformer 12 consists of approximately 2000 turns of fine wire.Neglecting saturation effects, the voltage step-up ratio of thetransformer 12 would result in a voltage across the secondary coil 22 ofapproximately 100 volts. Saturation reduces this voltage toapproximately 50 volts peak.

Operationally, when ignition of the gas flow through a gas valve isrequired, the aforementioned current flow of approximately 2 to 5amperes occurs through the primary coil 14 of the transformer 12 and theheater 16. This current flow results in a voltage drop of approximately500 millivolts across the primary coil 14 of the transformer 12 causingan AC voltage of approximately 50 volts peak to be produced across thesecondary coil 22. This voltage has a positive and negative spikedconfiguration as shown in FIG. 2. When this voltage is applied to thediode 24, which acts as a half-wave rectifier, all negative voltagespikes are eliminated, and the capacitor 26 acts as a smoothingcapacitor. The resulting DC voltage level has a relatively low ripplefactor. The zener diode 28 acts as a clamp preventing this DC voltagelevel from rising above a predetermined value (usually about 25 volts).The output of this circuit 10 is applied to a start circuit which maytake one of many different forms and is operable after it has receivedpower for a predetermined time, i.e., after the expiration of thepredetermined period of time, the start circuit actuates a relay which,in turn, operates the gas valve permitting gas to flow therethrough. Theresistance in series with this relay can be adjusted so that the relayoperates only if the voltage across the heater 16 is at leastapproximately 90 to 95 volts AC RMS. In this manner, sufficient voltagemust be applied to the heater 16 for the predetermined period of time inorder to heat same, thus preventing the gas valve from opening unlessthe heater 16 has reached ignition temperature. Thus, current throughthe heater 16 is effectively operating the timing circuit which, inturn, operates the gas valve through the relay. In summary, the circuit10 is fail safe in that it requires the application of sufficientvoltage to the heater 16 for a predetermined period of time, resultingin sufficient current flow through the heater to cause it to reachignition temperature prior to the opening of the gas valve. Anadditional check on voltage supplied to the heater uses a resistanceconnected in series with the relay which provides power to the heater.This resistance is adjusted so that the heater relay operates only ifthe voltage to the heater is at least approximately 90 to 95 volts ACRMS.

This circuit 10 overcomes the aforementioned problems associated withthe prior art. For example, it does not rely upon the visual detectionof a spark which is extremely difficult in the presence of ambientlight. Furthermore, it does not depend upon the acoustic recognition ofa spark which is difficult to achieve because of external noise and theproblems associated with recognizing a peculiar sound associated withthe spark. In addition, it does not rely upon proving the existence ofenergy pulses in the spark generating circuit which are possible withoutan actual spark. And lastly, it does not require measuring theelectrical resistance of a heater-type ignitor which has inherentinaccuracies due to the effects of aging on the circuit components andwhich requires relatively complex circuitry for fail safe operation.

Referring now to FIG. 3, the present invention is illustratedschematically in an electrical circuit 100 which incorporates a startcircuit, a gas solenoid valve and a flame rectification circuit. Thosecomponents which are similar to the components in FIG. 1 have likereference numerals and will not be discussed further. The timing circuitincludes resistors 102, 104, and 106; programmable unijunctiontransistor 108; capacitor 110; diode 111; and resistors 112 and 114arranged and interconnected as shown. The output from resistor 114 isconnected to the gates of field-effect transistors 116 and 118, and arelay coil 120 is connected in parallel with the transistors 116 and118. The common contact associated with the relay coil 120 is connectedto the input terminal 18 via a thermostat 122 and, upon actuation of therelay coil 120, connects the 117 volt AC source to a gas solenoid valve124.

The input terminal 128 is connected to a metallic probe or flameelectrode which is immersed in the burner flame. The equivalentelectrical circuit of the flame is shown generally by the numeral 130and is comprised of a resistor 132 connected in parallel with the seriescombination of a diode 134 and another resistor 136. A capacitor 138 isconnected to the AC input terminal 18 via thermostat 122 and to theinput terminal 128 of the probe. The input terminal 128 is alsoconnected to the gates of the field effect transistors 116 and 118 via aresistor 140. In addition, a capacitor 142 is connected to the commonside of the secondary coil 22 of the transformer 12 and to the gates ofthe field-effect transistors 116 and 118. The transformer common and theneutral side of the AC line are connected to chassis ground through aresistor 144 having a value of approximately one megohm.

Half-wave rectified DC power is provided to the relay coil 120 via adiode 146 and resistors 148 and 150. Resistor 148 can be varied toadjust the resulting voltage applied to the relay coil 120. A ripplesmoothing capacitor 151 is connected to the relay coil 120 and to thetransformer common.

The electrical circuit 100 operates in the following manner. When thethermostat 122 "calls" for heat, its contacts close which results in theclosing of contact 152 through another relay which is shown in FIG. 4.The closing of contact 152 causes a current of approximately 2 to 5amperes to flow through the primary coil 14 of the transformer 12 andthe heater 16. This current flow results in an AC voltage ofapproximately 50 volts peak being produced across the secondary coil 22of the transformer 12. The diode 24 acts as a half-wave rectifier andcapacitor 26 acts as a smoothing capacitor resulting in a DC voltagelevel having a relatively low ripple factor. The zener diode 28 preventsthis DC voltage level from rising above a predetermined value, typicallyabout 25 volts. This DC voltage is applied to the timing circuit. Theresistors 102 and 104 act as a voltage divider to bias the gate of theprogrammable unijunction transistor 108. Typical resistance values forthe resistors 102 and 104 are 2 megohms and 15 megohms, respectively,which "set" the gate of the transistor 108. Thus, the transistor 108remains unactuated until the capacitor 110 is nearly fully chargedthrough the resistor 106 and diode 111. The values for the capacitor 110and the resistor 106 may be chosen so that the charging time for thecapacitor 110 is relatively long. When the voltage at the anode of thetransistor 108 exceeds its gate voltage, the transistor 108 turns "on",effectively grounding the positive plate of the capacitor 110, i.e., theplate connected to the anode of the transistor 108. This groundingaction causes the capacitor 110 to apply a sufficiently negative voltageto the gates of the field effect transistors 116 and 118 through theresistor 114, turning these transistors "off", i.e., these transistorsusually act as a short circuit, but when there is a sufficient negativevoltage applied to their respective gates, they become essentially anopen circuit. The extinguishing of these transistors 116 and 118 causesthe relay coil 120 to become actuated which, in turn, causes the gassolenoid valve 124 to become actuated. It should be noted that relaycoil 120 will not become actuated unless sufficient voltage is appliedthereto and resistor 148 is adjusted so that relay coil 120 will notbecome actuated unless the voltage across the heater 16 is in excess ofa set value, for instance, 90 to 95 volts AC, in this example. Thus,sufficient voltage must be applied to the heater 16 for a predeterminedperiod of time ensuring that the heater 16 has reached ignitiontemperature before the gas solenoid valve 124 is allowed to open.

As soon as transistor 108 turns "on", the capacitor 110 begins todischarge through the transistor 108 and the resistor 112. The dischargetime may take approximately 5 seconds, for instance, to reduce thevoltage at the gates of the field-effect transistors 116 and 118 to alevel at which the transistors 116, 118 may again turn "on". During thistime the gas continues to flow to the burner. If the gas is not ignitedby the heater during this 5 second ignition period, then the fieldeffect transistors 116 and 118 again turn "on" which causes thedeactuation of the relay coil 120 and gas solenoid valve 124. Thisprocess is known as a five second trial for ignition period and can beset within wide limits by using different values for capacitor 110 andresistor 112.

If the gas is ignited during the foregoing 5 second ignition period, theflame acts as a low quality diode, shown schematically as the diode 134and resistors 132 and 136, from input terminal 128 to ground potential.This action as a diode causes the capacitor 138 to be charged so thatits bottom plate is negative with respect to its top plate. Thischarging action also causes the capacitor 142 to be charged through theresistor 140 so that its bottom plate is also negative with respect toits top plate which causes the field-effect transistors 116 and 118 tobe turned "off". This charging action ensures that the field-effecttransistors 116 and 118 remain turned "off", when there is a flame, eventhough capacitor 110 becomes discharged. Thus, the gas solenoid valve124 remains actuated permitting continuing gas flow to the burner. Theelectrical circuit 100 remains in this state as long as the thermostat122 is "calling" for heat and there is a flame. If the contactsassociated with the thermostat 122 open, upon their reclosure theforegoing ignition sequence is recommenced.

Once a flame has been established, the heater 16 is turned off by meansof a circuit (not shown). It should be noted that in some installationsthe heater is then used as a flame probe. In any event, if there is aninterruption in the gas flow to the burner or if the flame isextinguished due to a gust of wind, the voltage at capacitor 142 quicklydischarges through resistors 114 and 112 and the relay 120 becomesdeactuated closing the gas solenoid valve 124. The foregoing sequencereactuates the heater 16 through contact 152 and the entire sequence isreinitiated, i.e., current through the heater 16 again flows through theprimary winding 14 of the transformer 22. This provides power to the gasvalve timing circuit as upon initial start-up conditions. The circuitfor reactuation of the heater is not shown herein.

Referring now to FIG. 4 which is a partial schematic of an actuationcircuit for the heater 16, when the thermostat 122 "calls" for heat, thebase of transistor 260 receives a signal through a circuit (not shown)and actuates relay 205 connecting the 117 volt AC power source to theheater 16. The contacts associated with relay 205 are shown as contact152 in FIG. 3 and normally open contact 153 in FIG. 4. As an additionalrequirement for sufficient voltage to provide sufficient ignitiontemperature at the heater, resistor 202 may be adjusted so that relay205 is actuated only if the power source voltage is sufficient, i.e., atleast approximately 90 to 95 volts AC RMS, for this example. Thisadjustment ensures sufficient voltage at the start of the heating cycle,and the adjustment of resistor 148 in FIG. 3 ensures sufficient voltageat the end of the heating cycle.

Depending upon the specific circuit design, transistor 260 is turned offduring the programmed trial period or upon flame establishment. If theflame is lost due to an interruption in gas flow or a gust of wind,transistor 260 is turned on again initiating another timing cycleidentical to that which occurred upon thermostat closure. This circuitdetail is not shown herein.

The foregoing apparatus can be applied to all heaters operated from anAC power source. The heater may operate from 24, 117 or 240 volts AC.The thermostat may operate at a different voltage from the heater. Forexample, the thermostat may be operable at 24 volts AC and the heater at117 volts AC. In this case, the thermostat 122, as shown in FIG. 3,would simply be connected to the ungrounded side of a 24 volt AC powersource in order to make it operable at this latter voltage.

Certain modification and improvements will occur to those skilled in theart upon reading the foregoing. It should be understood that all suchmodifications and improvements have been deleted herein for the sake ofconciseness and readability, but are properly within the scope of thefollowing claims.

I claim:
 1. Apparatus for controlling the operation of a valve whichregulates the flow of fuel to a burner comprising:means for igniting theflow of fuel emanating from the burner, said igniting means beingresponsive to the flow of electrical current therethrough; timing meansoperable upon the expiration of a predetermined period of time ifelectrical current is flowing through said igniting means upon theexpiration of said pre-determined period of time; and means fortransforming the flow of electrical current through said igniting meansinto a voltage sufficient to cause said timing means to operate causingthe actuation of the valve permitting the flow of fuel to the burner,said transforming means being electrically connected to said ignitingmeans and to said timing means.
 2. The apparatus as defined in claim 1further including first switching means electrically connected to saidtiming means, operation of said timing means causing the actuation ofsaid first switching means and the valve permitting the flow of fuel tothe burner.
 3. The apparatus as defined in claim 2 wherein said firstswitching means is operable in response to the application of a firstpre-determined minimum voltage thereto.
 4. The apparatus as defined inclaim 1 further including second switching means electrically connectedto said igniting means, operation of said second switching meanspermitting the flow of electrical current to pass through said ignitingmeans causing the heating of said igniting means to the ignitiontemperature of the fuel.
 5. The apparatus as defined in claim 4 whereinsaid second switching means is operable in response to the applicationof a second pre-determined minimum voltage thereto.
 6. Apparatus forcontrolling the operation of a valve which regulates the flow of fuel toa burner comprising:means for igniting the flow of fuel emanating fromthe burner, said igniting means being responsive to the flow ofelectrical current therethrough; timing means operable upon theexpiration of a predetermined period of time if electrical current isflowing through said igniting means upon the expiration of saidpre-determined period of time; first switching means electricallyconnected to said timing means, operation of said timing means causingthe actuation of said first switching means and the valve permitting theflow of fuel to the burner; and means for transforming the flow ofelectrical current through said igniting means into a voltage sufficientto cause said timing means to actuate said first switching means and thevalve permitting the flow of fuel to the burner, said transforming meansbeing electrically connected to said igniting means and to said timingmeans.
 7. The apparatus as defined in claim 6 wherein said firstswitching means is operable in response to the application of a firstpre-determined minimum voltage thereto.
 8. The apparatus as defined inclaim 6 further including second switching means electrically connectedto said igniting means, operation of said second switching meanspermitting the flow of electrical current to pass through said ignitingmeans causing the heating of said igniting means to the ignitiontemperature of the fuel.
 9. The apparatus as defined in claim 8 whereinsaid second switching means is operable in response to the applicationof a second pre-determined minimum voltage thereto.
 10. Apparatus forcontrolling the operation of a valve which regulates the flow of fuel toa burner comprising:means for igniting the flow of fuel emanating fromthe burner, said igniting means being responsive to the flow ofelectrical current therethrough; timing means operable upon theexpiration of a predetermined period of time if electrical current isflowing through said igniting means upon the expiration of saidpre-determined period of time; first switching means electricallyconnected to said timing means, operation of said timing means causingthe actuation of said first switching means and the valve permitting theflow of fuel to the burner; means for transforming the flow ofelectrical current through said igniting means into a voltage sufficientto cause said timing means to actuate said first switching means and thevalve permitting the flow of fuel to the burner, said transforming meansbeing electrically connected to said igniting means and to said timingmeans; and second switching means electrically connected to saidigniting means, operation of said second switching means permitting theflow of electrical current to pass through said igniting means causingthe heating of said igniting means to the ignition temperature of thefuel.
 11. The apparatus as defined in claim 10 wherein said firstswitching means is operable in response to the application of a firstpre-determined minimum voltage thereto.
 12. The apparatus as defined inclaim 10 wherein said second switching means is operable in response tothe application of a second pre-determined minimum voltage thereto.